In conventionally known adaptive modulation communication, estimation of the state of the propagation path is performed to be a criterion by which to adaptively select a modulation parameter (MCS: Modulation and Coding Scheme) comprised of a plurality of modulation schemes, coding rates of error correction or a combination thereof.
Generally, the state of the propagation path is estimated using a preamble that is a known signal inserted into a radio frame, as a value of Carrier to Noise power Ratio (CNR) from the reception state of the preamble (see Non-patent Document 1). Further, in subcarrier adaptive modulation communication in multicarrier transmission schemes, a propagation path state for each subcarrier is estimated, and a modulation parameter is selected for each subcarrier.
FIG. 13 is a block diagram showing an apparatus configuration example in a conventional OFDM (Orthogonal Frequency Division Multiplexing) subcarrier adaptive modulation system. In FIG. 13, a propagation path measuring circuit 71 estimates a propagation path state from information of a received signal, and outputs an estimation result to a modulation parameter selecting circuit 72. The modulation parameter selecting circuit 72 (uniquely) selects a modulation parameter from the input estimation result of the propagation path state, and outputs information of the selected modulation parameter to a subcarrier adaptive modulation coding circuit 73.
The subcarrier adaptive modulation coding circuit 73 performs adaptive modulation for each subcarrier corresponding to the selected modulation parameter. An IFFT circuit 74 performs inverse Fourier transform on an output of the subcarrier adaptive modulation coding circuit 73 to transform into a digital OFDM signal, and outputs the signal to a D/A conversion section 75. The D/A conversion section 75 converts the digital OFDM signal to an analog signal.
Meanwhile, when a moving speed of a terminal increases, since temporal variations in the propagation path cannot be neglected, as a propagation path estimation method tolerant of temporal variations in the propagation path, a method is proposed for performing propagation path estimation by extrapolating propagation path measurement results using a plurality of past pilot symbols (see Non-patent Document 2).
FIG. 14 is a diagram illustrating an outline of the propagation path estimation method as described in Non-patent Document 2. Referring to FIG. 14, the propagation path estimation method using linear extrapolation processing in the above-mentioned technique will briefly be described below. In FIG. 14, hn-1 and hn are respectively assumed to be propagation path estimation results using a previous n−1th pilot symbol 81 and nth pilot symbol 82. The propagation path estimation value h^ in an estimation target symbol 83 is expressed as in Equation (1), using a time interval Tf87 between the n−1th pilot symbol 81 and nth pilot symbol 82, and a time interval Ti88 between the nth pilot symbol 82 and the estimation target symbol 83.
                    [Eq. 1]                                                                      h          ^                =                                                                              T                  f                                +                                  T                  i                                                            T                f                                      ⁢                          h              n                                -                                                    T                i                                            T                f                                      ⁢                          h                              n                -                1                                                                        (        1        )            
The aforementioned method is linear extrapolation using a linear equation, and further, a method is known of using extrapolation processing using a polynomial of a plurality of past propagation path estimation results.    Non-patent Document 1: Suzuki et al. “An Estimation Scheme of Propagation Path Characteristics for an Adaptive Modulation System” IEICE Technical Report, RCS94-65, p. 37-42, 1994    Non-patent Document 2: Yoshiki et al. “Performance of the OFDM based Adaptive Modulation System with a Multilevel Transmit Power Control Scheme using Delay Profile Extrapolation Technique for High Mobility Terminals”, IEICE Technical Report, RCS2002-58, p. 41-46, 2002